The field of the invention is nuclear magnetic resonance (“NMR”) imaging methods and systems. More particularly, the invention relates to the display of NMR image data indicative of flow, or motion over an acquisition time period to facilitate diagnosis of disease.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
MR methods have been developed that encode motion into the phase of the acquired signal as disclosed in U.S. Pat. No. Re. 32,701. These form a class of techniques known as phase contrast (PC) methods. Currently, most PC techniques acquire two images, with each image having a different sensitivity to the same velocity component. Images are then obtained by forming either the phase difference or complex difference between the pair of velocity-encoded images. Phase contrast techniques have been extended so that they are sensitive to velocity components in all three orthogonal directions, and the technique has been extended to so-called projection reconstruction MRI as disclosed in U.S. Pat. No. 6,188,922.
The “Chiari Malformation” is a birth defect occurring in around 1 in 2000 births. Referring particularly to FIG. 5, it is characterized by the descent of part of the cerebellum (the ‘tonsils’ 10) through the foramen magnum. The foramen magnum is the largest opening into the cranial vault at the base of the skull. The spinal cord, vertebral arteries and their anterior and posterior spinal branches enter the skull through this opening. The degree of descent of the tonsils 10 ranges up to 15 mm or more, with 3 mm generally being considered the point at which the patient is considered to have a malformation. (Such a condition is known as a Chiari I). Roughly half of the population with a Chiari malformation never exhibit any symptomatology. The onset of symptoms usually ranges from infancy to the mid-20's, although older patients developing symptoms is not uncommon.
The symptomatic malformation is characterized by damage to the upper spinal cord, with a syrinx (a cavity within the cord) developing about half the time. Because both the cranium and spine are rigid structures, blood pumped into the brain displaces the same volume of CSF down into the spine. Conversely, when blood leaves the brain during diastole, CSF returns to the cranium. Current theory holds that because the tonsils 10 obstruct part of the subarachnoid space, CSF flow is impeded, resulting in higher pressures within the spinal cavity. One variation on this theory is that the tonsils 10 act in the manner of pistons, creating pressure waves, which damage the spinal cord. In any event, higher than normal pressure have been measured in symptomatic Chiari patients with a manometer inserted into the spinal cavity.
Treatment is surgical, usually involving removing the tonsils 10 and widening the posterior part of the foramen magnum opening to allow greater CSF flow. With the advent of MRI, diagnosis of the degree of the descent of the tonsils 10 is easily measured, and this measurement is the current marker used for diagnosis. However, there seems to be very little correlation between the degree of tonsil descent and the severity of symptoms, nor has it been possible to predict the outcome of corrective surgery based on such anatomic MR images. In fact Chiari-like spinal cord damage occurs even with no descent of the tonsils at all. This condition is called a Chiari 0. Symptoms improve or disappear with the same surgical procedures used in Chiari I patients.
It seems reasonable to conclude that occlusion or blockage of flow caused by the tonsils 10 should lead to abnormal CSF flow velocities, and that diagnostic techniques based on measurement of CSF flow velocity might lead to better results. Since 1991, there have been roughly 30 studies using phase contrast MR to measure instantaneous CSF flow velocity and volume over the course of the cardiac cycle. These studies have yielded inconsistent and often contrary results. These have arisen due to uncorrected aliasing, venous or arterial flow being mistaken for cerebrospinal fluid flow, and the universal practice in these studies of determining flow parameters by averaging data for all CSF velocities at a particular point in time.
More recently, the peak CSF flow velocity at locations in the foramen magnum was used as an index for evaluating patients as described by Haughton et al “Peak Systolic And Diastolic CSF Velocity In The Foramen Magnum In Adult Patients With Chiari I Malformations And In Normal Control Participants”, American Journal of Neuroradiology, 24:169-176, February 2003. Bernoulli's principle states that if a given (incompressible) volume of fluid moves from an area of large cross-section to an area of smaller cross-section, the velocity will increase. Thus, it would seem reasonable that Chiari I patients, in which the cross-section of the subarachnoid space is decreased due to the presence of tonsils 10 should exhibit higher velocities then normal patients. Haughton et al measured the extreme velocity by inspecting individual voxels, yielding peak velocity measurements at each point in the cardiac cycle that was measured. In general, Chiari I patients did show higher extreme localized velocity measurements, but this was not a universal finding, mainly due to undetected aliasing in the majority of the patients which had the effect of lowering the measured extreme velocities.